The invention relates to an exposure control circuit of a camera, and more particularly, to such circuit which uses an analog semiconductor switch as a trigger switch for the exposure control circuit.
As is recognized, an exposure control circuit of a camera utilizes a photoelectric transducer element such as a silicon photodiode in order to derive a photocurrent which is proportional to the magnitude of light from an object being photographed. The photocurrent is then converted into a integrated voltage, which is then compared against a reference voltage to determine an exposure period in accordance with the intensity of the light from an object being photographed.
As a refinement, an output voltage from the silicon photodiode may be applied to an operational amplifier having a high input impedance in order to derive an integrated voltage which is free from the influences of other peripheral circuits. An exposure control circuit thus formed is driven by a trigger switch which is operated in connection with a shutter release operation of a camera.
The trigger switch which triggers the exposure control circuit of the camera heretofore comprises a switch having mechanical contacts which are disadvantageously subject to deterioration of their insulation or to poor contact in a high temperature or high humidity environment. It should be noted that an exposure control circuit which comprises a silicon photodiode and an operational amplifier is generally very sensitive and can be influenced by a leakage current of a very small magnitude. Hence, a very precise design, i.e. one having, for example, a very close tolerance against leakage current, as well as a very effective insulation, is required. Finally, the trigger switch must be fully capable of turning a very small current on and off.
However, a switch utilizing mechanical contacts cannot meet these requirements. In a high temperature or high humidity environment, the insulation may be degraded or the resistance of contact of mechanical contacts may increase, causing a poor contact which in turn results in variations in the accuracy of the automatic exposure of the camera with varying environment. In addition, the trigger switch utilizing mechanical contacts incorporates an expensive ceramic printed board, resulting in the complexity of the construction and a high cost.
To overcome the described disadvantages of the trigger switch which utilizes mechanical contacts, there has been a proposal to use a semiconductor switch such as a CMOS transmission gate switch employing a CMOS transistor, for example, as the trigger switch. While the use of such switch avoids the disadvantages of the trigger switch utilizing the mechanical contacts, the insulating resistance across the switch when it is turned off is low as compared with the resistance of the corresponding mechanical switch, so that a leakage current may flow through the switch. Such leakage current prevents a reliable operation of the exposure control circuit.